The present invention generally relates to modulation and demodulation in digital data transmission, and more particularly to a modulation and demodulation technique capable of dealing with multiple types of modulation based on hierarchical modulation.
In recent radio communications, digital modulation techniques are employed in, for instance, cellular phone networks, BS television broadcast, and other digital communication networks. Known modulation techniques used in digital mobile communications include quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM).
Quadrature phase shift keying (QPSK) is a modulation technique that uses two carriers out of phase by 90 degrees (that is, quarternary signal states), allowing 2 bits/symbol transmission. FIG. 1 is a QPSK signal constellation, in which the in-phase component and the guadrature component of the QPSK modulation signal are expressed in the phase plane defined by the I-axis (representing the real number) and the Q-axis (representing the imaginary number). The signal constellation is also called “signal diagram”. In QPSK modulation, modulation signals with four different phases are arranged at the vertexes of a square. In other words, the transmission signal is represented by one of the four phases.
Another type of QPSK modulation is pi/4-shift QPSK, which is derived from the above-described QPSK and used in personal digital cellular (PDC) phones based on the second generation mobile communications standard. With pi/4-shift QPSK, the phase of the carrier rotates by pi/4 per symbol, providing quarternary signal states.
In W-CDMA in FDD mode, which is one of the radio transmission protocols used in the third generation cellular systems based on the International Mobile Telecommunications-2000 (IMT-2000) standard, BPSK (binary phase shift keying) is used for uplink data modulation, while QPSK is used for downlink data modulation. In TDD mode, QPSK is employed for data modulation on both uplink and downlink.
Another known modulation technique is 16-QAM (quadrature amplitude modulation), which allows 4 bits/symbol data transmission using a combination of phase and amplitude of the carrier representing one of sixteen (16) four-bit patterns. FIG. 2 is an example of a 16-QAM signal constellation. With 16-QAM, four bits (16 patterns) of an input sequence are divided into four 2-bit patterns. The two carriers out of phase by 90 degrees are amplitude-modulated in the respective four signal states, and synthesized. The constellation plotted after the amplitude modulation is shown in FIG. 2. The divisions on the I-axis and the Q-axis are −0.9487, −0.3162, 0.3162, and 0.9487. Comparing the 16-QAM with the above-described QPSK under the same transmission rate, the occupied band-width of 16-QAM is narrower than that of QPSK. This means that 16-QAM is more effective to realize high-speed digital data transmission. However, 16-QAM is easily influenced by fading, which is the phenomenon of sharp fluctuation in intensity level of a radio wave due to environmental change including time and the distance between the transmitter and the receiver. The 16-QAM is mainly used for digital MCA (telecommunications business).
Since the above-described two modulation techniques, QPSK and 16-QAM, are used in digital mobile telecommunications, hardware capable of dealing with these two modulation techniques is desirable. Such hardware is advantageous for both users and manufacturers, reducing the cost of components and the space for accommodating the hardware, while improving operability. From this standpoint, JPA 9-27426 proposes a demodulation technique for demodulating both QAM modulation signals and QPSK modulation signals in one system.
However, in spite of the existence of multiple modulation techniques used in digital telecommunications, the transmission ability and the receiving ability are not always consistent with each other. If a receiver is capable of dealing with only QPSK modulation signals, 16-QAM modulation signals transmitted from a transmitter cannot be demodulated. To overcome this problem, a 16-QAM demodulator has to be furnished in the receiver, in addition to the QPSK demodulator, or alternatively, a system proposed in JPA 9-275426 is required. The former method causes the circuit scale and the cost to increase because an extra demodulator has to be added to the receiver. The latter method provides two independent signal paths, one functioning as a QAM demodulator when receiving a QAM modulation signal, and the other functioning as a QPSK demodulator when receiving a QPSK modulation signal. One of these two demodulation types is selected depending on the modulation type of the received signal. Accordingly, it is essential to furnish the functions of a QAM demodulator and a QPSK demodulator in the circuit structure.